Every day, hundreds of people are injured while operating power equipment of various kinds. Numerous systems have been proposed to make operation of power equipment safer. The simplest systems are guards that physically block the user from reaching dangerous portions of the equipment. Such guards are most effective around chains, belts and other portions of the drive system that can be made inaccessible without impairing operation of the machine. However, the most dangerous portion of power equipment is often the portion into which a user must feed material for processing. It is not possible to block access to such areas while still allowing the user to operate the machine. Therefore, a different form of protection must be provided for operational zones.
To protect the user against accidental injury in operational zones machines have been equipped with various systems to detect the presence of a user, or a user's extremity, in a dangerous area and take some precautionary action. For instance, some systems require a user to place each hand on a switch remote from the danger zone before the machine will operate. With proper placement of the switches, the user cannot have their hands in the danger zone and still actuate the machine. Unfortunately, users often bypass the switches and are injured as a result. Other systems utilize a radio frequency field to detect the proximity of a user to the operation area and inhibit operation if a human presence is detected. Still other systems utilize a conductive glove worn by a user. When the glove comes into contact with a dangerous element of the machine, the machine is stopped to minimize or eliminate injury.
All such systems suffer from a number of drawbacks, particularly when used on woodworking equipment. First, in woodworking equipment, the user often must move or hold the work piece during the processing operation. As a result, the user's hands must pass close to the cutting tool. Therefore, systems that rely on proximity detection are not sufficiently reliable. Systems utilizing a conductive glove are generally inconvenient, and in the case of many pieces of equipment, would not provide sufficient protection if the machine is simply turned off when contact is detected.